Combined diffusion bonding and superplastic forming is an established technique for making structural components, particularly lightweight components requiring complex internal structure, from materials that exhibit superplastic properties at elevated temperatures. These materials are primarily titanium, aluminium and alloys of both these metals.
In established DB/SPF processes, for example see U.S. Pat. Nos. 5,143,276, 4,534,503, GB-2030480, GB-2129340, U.S. Pat. Nos. 4,607,783, 4,351,470, 4,304,821 and EP-0502620, it is known to apply stop-off material to selected areas of two or more sheets of superplastic material; several sheets, including the sheets to which stop-off material has been applied, are then assembled into a pack with the stop-off material lying between adjacent superplastic sheets. The assembled pack is then heated and compressed until the sheets are diffusion bonded together; however, the sheets are not bonded in the selected areas covered by stop-off material since the stop-off material prevents diffusion bonding in those areas. The superplastic forming step is then conducted by heating the bonded pack, usually in a mould, to a temperature at which the components exhibit superplastic properties. An inert gas is then injected in a controlled manner into the unbonded areas of the pack under high pressure so as to "inflate" the sheets gradually into a three dimensional structure having an outer shape corresponding to the shape of the mould. The configuration of the final composite structure is dependent upon, among other things, the number of sheets in the pack, the location of the stop-off material and the shape of the mould.
It is known, for example from GB-1495655, to form a composite panel from a pack comprising a pair of opposed face sheets and a core sheet sandwiched between, and bonded at selected points to, the face sheets; in the superplastic forming process, the face sheets are forced apart and because the internal core sheet is selectively attached to both of the face sheets, the core sheet adopts a zigzag shape that, in effect, constitutes struts extending from one face sheet to the other.
U.S. Pat. Nos. 4,304,821 and 5,143,276 each describes the making of a panel from four sheets of superplastic material from a pack comprising a pair of opposed face sheets and two core sheets sandwiched between the face sheets; the two core sheets are bonded to each other at selected points by linear welds. The face sheets are superplastically formed by injecting gas into the area between each face sheet and the adjacent core sheet to expand the face sheets into the shape of a mould; gas is then injected between the two core sheets. Because the core sheets are selectively joined by the linear welds, the core sheets expand to form cells extending between the face sheets; the side walls of the cells are formed by U-shaped doubled-back sections of the two core sheets.
The superplastically formed panels produced using these known techniques have many advantages but they are not suitable for withstanding localised high loads, for example where other external components will bear on or are to be attached to the panels.
EP-754098 proposes a process for superplastically forming a part for use as an aircraft component, in which localised pre-thinning of a sheet is employed to facilitate superplastic forming in areas where forming tends to be slow and thus to avoid excess thinning in other areas of the part. In this way, the overall thickness of the sheet can be controlled during forming and hence the strengthening of given areas of the final part is possible.
Another process for stiffening a superplastically formed panel is described in U.S. Pat. No. 4,632,296. In this process, the initial thickness of predetermined areas of the sheets to be formed is selected to control the rate of superplastic deformation of the sheet during forming. This can be used both to avoid areas of malformation and to produce reinforced areas of extra strength in the final panel.
Nevertheless, neither of these two prior references addresses the problem of providing in a superplastically formed panel localised areas capable of withstanding substantial point loads, for example where other components will bear on or require load bearing attachment to the panel.
Furthermore, none of the prior methods provides a superplastically formed panel suitable for machining, post forming, in order to enable the attachment of other components.
The present invention addresses these problems and seeks to overcome them by providing a superplastically formed structure that is more robust than prior art panels and that has localised load bearing areas.